Importance of baseline assessments: monitoring of Brunei River's water quality

Water quality has declined in nearly all major rivers in Africa, Asia, and Latin America (UN 2021). This decline in water quality has become a severe issue in many countries, and water quality monitoring is one of the top priorities for resource protection policy (Kachroud et al. 2019). A plethora of factors are causing river deterioration, such as industrialization, urbanization, and contemporary agricultural methods (Cullis et al. 2019; Sharma et al. 2020; Suhip et al. 2020). They significantly influence an area's hydrology, lowering surface and groundwater quality, and threatening aquatic life. River water quality deteriorates due to pollutants classifiable as point and non-point source pollution entering the surface water. Non-point and point source pollution affect surface water ecosystems, causing eutrophication, excessive algal growth, and chemical and microbial contamination (Suaad 2021). A distinction between the two types of pollution is that contaminants from point sources enter the water through an easily identifiable distinct route, such as effluent from sewage treatment plants. Pollutants from non-point sources, on the other hand, enter from unidentified diffuse sources and are difficult to control, such as stormwater runoff from agricultural land.

Baseline assessment involving water quality monitoring and identifying pollutant trends for protecting the environmental quality of rivers are key for establishing baseline conditions for comparative purposes (Arevalo-Mejía et al. 2016). Understanding surface water's physical, chemical, and biological characteristics aids in monitoring and managing factors contributing to water quality deterioration. Water quality is evaluated using a variety of parameters, and the choice of parameters is significantly impacted by the purpose or objective of any study (Uddin et al. 2021). Parameters such as turbidity, total dissolved solids (TDS), temperature, and electrical conductivity (EC) form the physical parameters. At the same time, chemical parameters comprise pH, biochemical oxygen demand (BOD), nitrogen, and chemical oxygen demand (COD), whereas total (TC) and fecal (FC) coliform bacteria are examples of biological parameters (Vinogradoff & Oliver 2015; Hassan 2020).

Water contamination is one of Brunei's concerns, particularly as improving the state of its water resources and ensuring their long-term viability are key development goals. Brunei's water use is among the highest in Southeast Asia, at 350 liters per day per person (DWS 2018). Most of Brunei's drinking water resources come from surface water (FAO 2011). Water quality studies in the region are limited (Goh 1991; Yusri et al. 2018; Hong et al. 2021; Azffri et al. 2022, 2023; Godeke et al. 2022). However, recent studies and concerns about surface water ecosystems, their state and quality, and the influence of climate change on the Brunei Rivers have intensified (Godeke et al. 2020; Shams et al. 2021). Goh (1991) examined the garbage output by Kampong Ayer households based on a sample survey and the environmental implications for Brunei River water quality. The author concluded that though the river quality has not deteriorated greatly over the years, action must be taken soon to dispose of the garbage of the whole settlement properly. In a subsequent study, Godeke et al. (2020) researched to explore whether potential shifts in climate, as reflected in rainfall data, could be associated with observed alterations in the water quality of the Tutong River. The results showed that a statistically significant rise in aluminum concentrations occurred during the years investigated (2014–2017). The authors further suggested that changes in pH triggered by rainfall intensities and groundwater levels contribute to the rise in aluminum concentration in the river water.

For future studies that aim to investigate water quality, baseline assessments are critical because they can offer a point of reference and comparison that allows the quantification or change in water quality over a period of time. Therefore, this study compares monitoring data of selected physiochemical and biological parameters (pH, temperature, ORP, turbidity, dissolved oxygen (DO), conductivity, TDS, salinity, ammonia-nitrogen, BOD, and total coliform) in 1984, 2019, 2020, and 2021 between a baseline study in 1984 and recent water quality assessments in 2019+. A data gap exists between 1984 and 2019 + ; no monitoring data is available. This study aims to establish the existing quality of the Brunei River based on selected parameters and identify possible factors causing the degradation of the river.

Brunei has a tropical climate with total annual rainfall exceeding 2,300 mm (Hasan et al. 2016), warm air temperatures of 28–32 °C, and high humidity of 70–98% (Chua et al. 1987). From October to January, the first rainfall maximum occurs, with December being the wettest month. The second minor maximum occurs from May to July, with May being the wettest month. The lowest minimum occurs from late January to March, followed by a slight minimum from June to August.

The Brunei River, along with its tributaries (Butir, Damuan, Kedayan, and Kianggeh), is one of Brunei Darussalam's major rivers. The 41-kilometer-long mangrove-lined Brunei river flows from the Brunei-Muara district around 2 km away from the Malaysian border in the northeast direction (Syed 1987) draining into Brunei Bay. In the north of the Brunei-Muara district surface and groundwater flow toward the South China Sea (Azhar et al. 2019). The Brunei River that feeds the bay varies in depth from 4 to 26 m and has a far lower freshwater flow than the two main rivers Sungai Temburong and Sungai Limbang (Chua et al. 1987). The catchment area surrounding the study area is primarily used for residential, mining, commercial, and agricultural purposes, all within Bandar Seri Begawan, the capital city. The Brunei River as a gaining river is also the shortest and most polluted significant river in Brunei (Yau 1991). Geologically, the catchment area is generally mapped as alluvium underlain by the Belait Formation (Syed 1987; Sandal 1996). The Belait Formation consists of interbedded sandstone, shale, and clay deposited in littoral to deltaic conditions. The shale and clay are generally gray and maybe sandy and silty. The major part of Brunei-Maura is situated on geologically recent peat deposits. However, the hills surrounding the area consist of sandstone and shales.

Kampong Ayer is a traditional Brunei village built on stilts above the river. A substantial number of domestic contaminants and other solid waste are discharged into the Brunei River, primarily from Kampong Ayer and other small settlements scattered along the river and its tributaries (Syed 1987; Goh 1991). Furthermore, the river receives a substantial percentage of stormwater runoff and treated and untreated sewage from Bandar Seri Begawan, the largest urban center (Syed 1987). The Brunei river pollution persists despite numerous attempts to control it, including a BND 3.9 million donation by the Brunei government for Sungai Brunei cleanup in 2006, a BND 66 million investment in the Sungai Akar Wastewater Treatment Plant (Bakar 2018), and the ‘River Clean-Up Operation (RECOVER)’, ‘Save Kampong Ayer’, which saw the removal of 20,000 bags of refuse from the river in 2 months.

On the other hand, the capital city sewerage system operates efficiently during storms, since sewerage and storm runoff are separated. Septic tank systems serve certain residential houses but also major shopping malls which are not currently connected to the centralized sewerage system. The predominantly clayey soils in the study area do not generally allow soakaways, and septic tank outlets are connected directly to monsoon drains which ultimately drain into the river. In many places, monsoon drains are neglected and frequently clogged with silt creating anaerobic conditions and odor issues (Syed 1987; Chamara & Koichi 2017). As a result of the large flows that the monsoon drains can produce during storms, accumulated septic tank effluent and other debris are flushed into the river at a relatively high rate. The river is also frequently utilized for water transportation by commercial and artisanal fishermen. The river's significance as the backdrop for Bandar Seri Begawan's metropolis is also vital in terms of esthetics.

The Aqua TROLL 600 (InSitu) Multiparameter Sonde was used to monitor water parameters in-situ such as pH, temperature, ORP, ammonia-nitrogen, TDS, EC, and salinity. All parameters were measured at depth intervals of 2–5 m. Turbidity and BOD were studied in 2020 and 2021, while the other nine parameters (pH, temperature, ORP, DO, conductivity, TDS, salinity, ammonia-nitrogen, and total coliform) were examined in 1984, 2019, 2020, and 2021. Two to three data points were frequently obtained at each monitoring station to increase accuracy. Two water samples were collected from each station in a designated polyethylene bottle and kept in a cool box on ice during the sampling event. The samples were subsequently transferred to the laboratory and immediately analyzed. The HF scientific M100+ laboratory turbidimeter was used to test turbidity, while the Hanna model HI 2400 DO meter measured dissolved oxygen. The BOD was measured over 5 days (APHA 2003), whereas total coliform was quantified using a multiple-tube fermentation approach (APHA 1998) at the wastewater laboratory of the Brunei Government.

Box plots of the 11 individual physiochemical parameters were prepared for the years 1984, 2019, 2020, and 2021.

The hierarchical cluster analysis (HCA) technique was performed based on Ward's approach to group the 11 water parameters in 2019, 2020, and 2021 into clusters using the squared Euclidean distance, which is the most commonly used measure of distance (Suresh et al. 2017). Samples from the same cluster have high homogeneity, whereas samples from different clusters have high heterogeneity (Lin 2010; Oluwaniyi & Asiwaju 2020). Based on the clustering results, dendrograms are then built using the levels of similarity at which observations are combined.

A principal component analysis (PCA) is used when a number of parameters are correlated; it reduces the number of measured parameters to a smaller number of principal components that account for the majority of the observed parameter variance (El Aatik et al. 2023). The ultimate focus of PCA was to extract from a sizable body of data primary information indicative of the typical features of Brunei's River environment and represent it as a new set of independent variables of the principal component. PCA was chosen to extract principal factors that are similar to different sources of variation based on eigenvalues (Dieu et al. 2016); selected eigenvalue coefficients were greater than 1 (Yen et al. 2018). Scores and weighing factors (loading) are used to express the correlation between the principal components and the water quality parameters (Asowata et al. 2015; Feher et al. 2016). If the weighing factor's absolute value is greater than 0.75, there is a strong correlation between the main component and the water quality indicators. Between 0.75 and 0.5, there is a moderate correlation; and between 0.5 and 0.3, there is a weak correlation (Liu et al. 2003). In order to examine the effects of each water quality parameter and potential sources of pollution, PCA was used in this study to analyze the physical, chemical, and biological water quality parameters from eight monitoring stations in the study area (Figure 1) in 2019, 2020, and 2021.

The study area map (Figure 1) was produced using ArcGIS 10.8. R was used for the box plots, the statistical package for the social sciences (SPSS 25) was used for HCA, and Originpro 23 was utilized for PCA. The results were then compared to the Food and Agriculture Organization of the United Nations (FAO) standards as well as the United States Environmental Protection Agency (USEPA) guidelines, and Department of Environment (DOE): Malaysia National Water Quality Standards (NWQS) to interpret the quality of water in the Brunei River. These standards were chosen because Brunei Darussalam does not have its own water quality standards yet. The NWQS class I signify the conservation of the natural environment and highly sensitive aquatic species. Class IIA and IIB waters allow sensitive aquatic species to thrive and can be used for human recreational use. Class III is excellent for economically valuable fisheries, tolerant species, and livestock watering. Class IV can be used for irrigation and Class V is of no-good use. Table 1 summarizes the guidelines for irrigation (Ayers & Westcot 1985), surface water and aquatic life (USEPA 1986), and surface water (DOE 1994) applied for interpreting water quality for this study.

The EC values were slightly lower in the upstream monitoring stations compared to the high values in the downstream stations. Generally, the conductivity levels (Table 2) were higher than FAO and NWQS standards in the study area. This could be due to the influence of high tides (Zhamaletdinov et al. 2018), as well as watershed geology, wastewater from Kampong Ayer's vicinity, Bandar Seri Begawan city stormwater runoff, and untreated sewage effluent (Abha 2014).

Downstream station B recorded the lowest turbidity value of 4.5 NTU, while the highest value of 76.9 NTU was recorded in upstream station Q in 2020. Most concentrations are still within EPA standard permissible limits (<50 NTU) for aquatic life and agriculture except upstream station Q. The turbidity measurements also showed that the Brunei River is not good for highly sensitive aquatic species based on the NWQS standards. The spike in turbidity at station Q in 2020 could be due to dredging activities which occurred at the riverbanks near the quarry (Widana 2019). The impact of the COVID-19 pandemic (Callejas et al. 2021) lockdown in Brunei may have contributed to the stark difference in turbidity between the two years (2020 and 2021). TDS's highest value recorded over the 4 years was 29.5 ppt at station E in 1984, and the lowest value obtained was 5.1 ppt at station Q in 2021 (Figure 2). TDS is proportionally related to salinity and conductivity; in the 4 years studied, upstream stations had lower TDS levels than downstream sites. High TDS concentration in the downstream stations E, D, and B may be due to seawater intrusion (Wilopo et al. 2021). Anthropogenic activity, runoff with high suspended matter, and sewage effluent may all be contributing factors to the estuarine river’s high TDS values in the upstream stations (Hussain 2019). The DO cycle in surface water varies geographically and temporally since biochemical and hydroclimatic activities such as discharge and organic waste influence its concentration (Varanka & Hjort 2016). DO values have remained within the USEPA standard for surface water. However, excessive nutrient and organic matter inputs to rivers, such as from agricultural fields and domestic wastewater runoffs, can increase primary production and accelerate decomposition rates. These factors result in oxygen saturation deficits (Rosemond et al. 2015; Saltarelli et al. 2018), especially at high temperatures due to higher respiration rates and during darkness due to a lack of photosynthesis. The increase in BOD concentration in 2020 at station N could have been caused by wastewater (Muhammad et al. 2023) directly discharged into the river. Furthermore, rivers may be considered moderately polluted when the BOD levels are between 2 and 8 mg/l (Daoliang & Shuangyin 2019).

Ammonia-nitrogen, a crucial ingredient for plant growth in nature, poses a major threat to aquatic life and the environment when present in excess (Krisbiantoro et al. 2020). The increase in ammonia-nitrogen levels in the downstream stations in 2019 may have resulted from runoff of agricultural fertilizers, Kampong Ayer domestic waste, and Pintu Malim sewage works (Constable et al. 2003; Sanchez et al. 2023) due to increased rainfall intensity into water bodies. With the exception of 2019, ammonia-nitrogen levels were consistent with FAO and NWQS standards for industrial, agricultural, and aquatic life (Table 1). As expected, the highest total coliform value was recorded in the vicinity of Kampong Ayer at station J due to the direct discharge of effluents from Kampong Ayer and contamination of runoff by septic tank discharges (Yau 1991; Samia et al. 2016; Xie et al. 2022). The bacterial contamination of the river gives some concern due to the close proximity of the inhabitants of Kampong Ayer to the waters. The unexpected increase in concentration at station Q could have resulted from the direct discharge of fecal matter from nearby residences at Kampong Ayer. The presence of high coliform levels observed in stations J and Q may also suggest the existence of elevated nitrate concentrations at these stations (Ram et al. 2022).

The monitoring stations N and G are within the Kampong Ayer vicinity, which receives pollution from anthropogenic activities, land use, and wastewater. Station P is mainly dictated by the natural anoxic waters entering the river from the upstream mangrove swamps surrounded by a mangrove ecosystem (Kalu & Ogbonna 2021). The runoff from these places is typically low in DO, contains a significant number of debris, and is acidic (Ouyang 2005). The downstream stations E, D, and B, also in cluster 2, are closer to Brunei Bay and influenced by the Pintu Malim sewage works, commercial areas, and Kampong Sungai Matan village, respectively. Pollutant loads from point sources into the Brunei River include direct wastewater discharge from Kampong Ayer, quarry activities, and Pintu Malim sewage works. In contrast, the non-point sources in the study area include runoff from urban areas, forested areas, and agricultural land, which can be expected to enter a water body over a large area. Agricultural activities mainly relate to small-scale paddy fields upstream of the study area and market gardens. Additionally, upstream monitoring stations frequently have higher pollution levels than downstream. The results show that HCA is a valid multivariate technique for evaluating and categorizing river water quality.

Furthermore, wastewater treatment plants, failing septic systems in Kampong Ayer, and urban stormwater runoff can also cause a spike in turbidity, BOD, and coliform bacteria due to organic waste in water (Connor 2016). PC3 has moderately positive pH and temperature loadings (Figure 5), whereas DO has a weak loading. The Brunei River's aquatic life is directly impacted by these variables. Acidic pH in the upstream stations P and Q due to acidic and anoxic runoff from the quarry site and mangrove area may have led to the release of toxic heavy metals into the water (Wen et al. 2021; Azffri et al. 2022), which has a detrimental effect on aquatic life. The majority of aquatic organisms are adapted to live in a narrow temperature range, and they perish when the temperature rises or falls (Griffin et al. 1999) above a certain threshold. ORP loads are moderately positive in PC4 (Table 3 and Figure 5). ORP reactions control the behavior of many chemical constituents in natural waters and sewage (Viktoras et al. 2010). The presence of high levels of organic matter in the water, such as sewage from Kampong Ayer or agricultural runoff causing pollution, can decrease the ORP (Wetzel 1983; Wang et al. 2022) by consuming DO through the process of oxidation. Ammonia-nitrogen loads are positively strong in PC5. The direct dumping of domestic waste from Kampong Ayer into the river may have contributed to the rise in ammonia-nitrogen levels in the river over time (Constable et al. 2003). Overall, factors obtained from PCA indicated that the parameters responsible for variations in water quality are principally associated with seawater intrusion, agricultural activities, domestic wastewater, and surface runoff from villages and urban areas.

Water quality management requires baseline evaluation of environmental impacts, sources, and pollutants that cause significant damage in rivers. The Brunei River is affected by saltwater intrusion, hydrological processes, and anthropogenic activities.

Brunei often experiences high-intensity precipitation events causing urban and agricultural runoff that can carry high levels of pollutants into waterways. The mangrove swamp, which primarily surrounds estuaries, is one of the distinctive biotopes found in tropical nations like Brunei. Kristensen et al. (2008) discovered that runoff from mangroves can have high concentrations of organic matter and nutrients like nitrogen and phosphorus. The authors noted that nutrients and organic matter may stimulate the growth of phytoplankton and other primary producers in nearby waters, which may cause eutrophication. The increase in ammonia-nitrogen levels in the downstream stations may have been brought on by runoff of agricultural fertilizers used around farming areas surrounding the Brunei River, particularly in 2019. Heavy rainfall events resulting in overbank flooding can lead to the ingress of contaminants from agricultural soils and rivers (Jose et al. 2019).

Globally, seawater encroachment into surface water represents a substantial pressing concern. In addition to the anthropogenic activities that may increase TDS and salinity in the upstream stations, seawater intrusion can also substantially impact the TDS and salinity concentrations. Seawater intrusion affects terrestrial ecosystems, with ramifications extending to residential and agricultural water supplies and urban industrial processes (Zhou et al. 2012).

Saltwater intrusion becomes notably more critical in scenarios where sea level rise advances further upstream, resulting in modifications to salt-sensitive habitats and adversely affecting the development of local fauna and flora (Liu & Liu 2014). For example, when an estuarine river undergoes artificial deepening through dredging operations (see Figure 6), it amplifies the pressure gradient owing to increased depth, concurrently diminishing the influence of tidal currents on mixing, thereby exacerbating saltwater intrusion (Hoagland et al. 2020).

Southeast Asian coastal environments are under a variety of stresses, primarily from land-based human activities (Sasekumar 1980). Due to rapid population growth, increased urbanization, and industrialization, anthropogenic activities have projected upward over time. By extracting water for urban purposes, modifying biophysical structures, or replacing natural infrastructure to transfer and regulate the flow of water, humans are changing the hydrologic cycle as they build cities (Klove et al. 2014). Bandar's commercial districts near the river in locations N, J, and G can also modify the microclimate, slow down infiltration, and speed up runoff (Alberti 2008). Another influence on the Brunei River is the direct discharge of effluents and wastewater polluting the river in Kampong Ayer; accumulation of solid wastes under houses at low tide littering the shoreline contributes to pollution in both upstream and downstream stations. The increasing population around the Kampong Ayer floating village is also a major factor contributing to the degradation of river water quality. As the population increases, so does the amount of sewage and wastewater generated, which may lead to high levels of pollutants in rivers (Paul & Meyer 2001; Edokpayi et al. 2017; Kairunnisa et al. 2021). Total coliform bacteria in 2020 and 2021 doubled compared to 1984 (Figure 2). During this time, the population of Brunei also doubled (World Bank 2023). Most of the population is concentrated around the capital city through which the Brunei River flows.

Unfortunately, there were significant errors while measuring the nitrate concentrations using the Aqua TROLL 600 at Brunei River due to strong interference of ions such as chloride, bicarbonate, chlorate, nitrite, and sulfate. Therefore, future studies are required to fully comprehend the effects of nutrients, and heavy metals in the study area. By researching the effects of nutrients and heavy metals in rivers, we can better understand the environmental, health, and economic risks associated with these pollutants and create strategies to lessen their impact on Brunei water resources (Chheang et al. 2023).

The findings from the baseline assessments on Brunei River pollution underscore the pressing need for immediate and concerted action to address critical environmental challenges. The data and analyses presented reveal the alarming extent especially for total coliform bacteria to which the Brunei River is being contaminated by a myriad of pollutants, both from identifiable point sources and diffuse non-point sources. These pollutants pose not only a grave threat to the balance of aquatic ecosystems but also cast a shadow on the wellbeing of communities that rely on the river for their water supply and livelihoods. In 1984, 2019, 2020, and 2022, the parameter values pH, temperature, and DO measured in the 4 years are within the permissible limits of the FAO, EPA, and NWQS, aside from turbidity, EC, salinity, NH₃-N, TDS, and TC. Low ORP readings in this study suggested a reduced ability of the water to support aquatic life. BOD concentration in the study revealed that the Brunei River may be moderately polluted. It is concerning that the Brunei River exceeds the NWQS coliform bacteria fishing guideline values. The box plots demonstrated that total coliform bacteria in 2020 and 2021 are higher than 1984. This is likely due to increased urbanization, population growth, and anthropogenic activities surrounding the river over the years. HCA classified the eight monitoring stations into two clusters based on pollution severity and indicated upstream monitoring stations are more polluted in the study area due to the influence of the nearby quarry and the direct discharge of waste from Kampong Ayer. PCA reduced the 11 water quality parameters to five important principal components, which accounted for 75.21% of the total variance in the original data. PCA also revealed saltwater intrusion, hydrological processes, and anthropogenic impacts as possible causes of Brunei River water quality degradation.

The study's findings emphasize the importance of adopting a multifaceted approach that combines rigorous regulatory measures, comprehensive pollution monitoring systems, and robust public awareness campaigns. Further research should be done to analyze heavy metals concentration in the river. It is evident that pollution mitigation strategies must encompass not only curbing direct discharges from Kampong Ayer but also managing the runoff of pollutants from agricultural and urban landscapes. As we move forward, these findings should serve as a clarion call to policymakers, industries, and individuals alike in Brunei Darussalam. Concerted efforts are required to reduce pollution at its source, invest in advanced wastewater treatment technologies, and promote sustainable land use practices. Only through collaborative and determined action, we can aim to reverse the increase in total coliform bacteria revealed by this study and ensure that the Brunei River regains its vitality, serving as a life-nurturing artery for both nature and humanity.

The authors would like to thank the Brunei Darussalam Public Works (Jabatan Kerya Raya) for providing the 1984 data.

This research was funded by Universiti Brunei Darussalam, grant number UBD/RSCH/1.18/FICBF(b)/2022/004UBD/UGS/2022.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.